Our poll shows a close race.

Our poll result is about what was expected. But remember: It’s just one poll, and we talked to only 505 people. Each candidate’s total could easily be five points different if we polled everyone in the district. And having a small sample is only one possible source of error.

About the race

Dino Rossi is a former legislator with high name recognition from several statewide campaigns. 45% favorable rating; 39% unfavorable; 16% don’t know

Based on 505 interviews

This open seat has drawn national interest as a potential pick-up for Democrats.

This district has never sent a Democrat to the House, but it has regularly voted for Democrats in presidential races. Congressman Dave Reichert’s decision to retire dealt a big blow to Republicans’ chances of extending their streak. He even held the district through the 2006 and 2008 Democratic wave elections.

Mr. Rossi is considered one of the top Republican recruits of the cycle, and his bid is a major reason this is still considered a top-tier race. He has raised nearly $3 million.

Mr. Rossi has served as a state senator and has repeatedly sought statewide office, running for governor twice and the U.S. Senate once. He won this district each time.

Ms. Schrier is focusing on health care and has said the Republican effort to repeal the Affordable Care Act prompted her to run for office.

The district’s quirky boundary embraces affluent high-tech suburbs and Boeing facilities to the west and cherry and apple farms across the Cascade Mountains, and its business makeup leaves it vulnerable to the president’s actions on trade.

How our poll result changed

As we reach more people, our poll will become more stable and the margin of sampling error will shrink. The changes in the timeline below reflect that sampling error, not real changes in the race.

One reason we’re doing these surveys live is so you can see the uncertainty for yourself.

But sampling error is not the only type of error in a poll.

Our turnout model

There’s a big question on top of the standard margin of error in a poll: Who is going to vote? It’s a particularly challenging question this year, since special elections have shown Democrats voting in large numbers.

To estimate the likely electorate, we combine what people say about how likely they are to vote with information about how often they have voted in the past. In previous races, this approach has been more accurate than simply taking people at their word. But there are many other ways to do it.

Assumptions about who is going to vote may be particularly important in this race.

Our poll under different turnout scenarios

Who will vote?

Est. turnout

Our poll result

The types of people who voted in 2014

194k

Rossi +5

People who say they are almost certain to vote, and no one else

249k

Schrier +4

Our estimate

252k

Schrier +1

People whose voting history suggests they will vote, regardless of what they say

253k

Even

People who say they will vote, adjusted for past levels of truthfulness

270k

Schrier +1

The types of people who voted in 2016

312k

Even

Every active registered voter

430k

Schrier +1

All estimates based on 505 interviews

The types of people we reached

Even if we got turnout exactly right, the margin of error wouldn’t capture all of the error in a poll. The simplest version assumes we have a perfect random sample of the voting population. We do not.

People who respond to surveys are almost always too old, too white, too educated and too politically engaged to accurately represent everyone.

How successful we were in reaching different kinds of voters

Called

Inter-viewed

Successrate

Ourrespon­ses

Goal

18 to 29

4364

27

1 in 162

5%

9%

30 to 64

25879

320

1 in 81

63%

62%

65 and older

6110

158

1 in 39

31%

29%

Male

16105

242

1 in 67

48%

48%

Female

20254

263

1 in 77

52%

52%

White

28320

423

1 in 67

84%

80%

Nonwhite

4635

41

1 in 113

8%

11%

Cell

20917

278

1 in 75

55%

—

Landline

15442

227

1 in 68

45%

—

Based on administrative records. Some characteristics are missing or incorrect. Many voters are called multiple times.

Pollsters compensate by giving more weight to respondents from under-represented groups.

But weighting works only if you weight by the right categories and you know what the composition of the electorate will be. In 2016, many pollsters didn’t weight by education and overestimated Hillary Clinton’s standing as a result.

Even after weighting, our poll does not have as many of some types of people as we would like.

Here are other common ways to weight a poll:

Our poll under different weighting schemes

Our poll result

Don’t weight by primary vote, like most public polls

Schrier +5

Weight using census data instead of voting records, like most public polls

Schrier +4

Don’t weight by education, like many polls in 2016

Schrier +1

Our estimate

Schrier +1

All estimates based on 505 interviews

Just because one candidate leads in all of these different weighting scenarios doesn’t mean much by itself. They don’t represent the full range of possible weighting scenarios, let alone the full range of possible election results.

Undecided voters

About 9 percent of voters said that they were undecided or refused to tell us whom they would vote for.

Issues and other questions

We're asking voters about health care, and also about whether they support Brett Kavanaugh’s nomination to the Supreme Court.

Do you approve or disapprove of the job Donald Trump is doing as president?

Approve

Disapp.

Don’t know

Voters n = 505

45%

48%

7%

Would you prefer Republicans to retain control of the House of Representatives or would you prefer Democrats to take control?

Reps. keep House

Dems. take House

Don’t know

Voters n = 505

45%

47%

8%

Do you support or oppose Brett Kavanaugh’s nomination to the United States Supreme Court?

support

oppose

Don’t know

Voters n = 505

45%

43%

12%

Do you support the creation of a national insurance program, in which every American would get insurance from a single government plan?

Support

Oppose

Don’t know

Voters n = 505

52%

41%

7%

Do you support repealing and replacing the Affordable Care Act, also known as Obamacare?

Support

Oppose

Don’t know

Voters n = 505

48%

46%

6%

Do you or a member of your family have a pre-existing health care condition like asthma, heart disease or diabetes?

Yes

No

Don’t know

Voters n = 505

40%

58%

1%

Percentages are weighted to resemble likely voters.

What different types of voters said

Voters nationwide are deeply divided along demographic lines. Our poll suggests divisions too. But don’t overinterpret these tables. Results among subgroups may not be representative or reliable. Be especially careful with groups with fewer than 100 respondents, shown here in stripes.

Gender

Dem.

Rep.

Und.

Female n = 263 / 52% of voters

50%

40%

10%

Male 242 / 48%

42%

51%

7%

Age

Dem.

Rep.

Und.

18 to 29 n = 28 / 6% of voters

67%

27%

6%

30 to 44 90 / 22%

43%

44%

13%

45 to 64 229 / 43%

45%

48%

7%

65 and older 158 / 29%

44%

46%

10%

Race

Dem.

Rep.

Und.

White n = 426 / 83% of voters

45%

47%

9%

Nonwhite 66 / 14%

54%

37%

9%

Race and education

Dem.

Rep.

Und.

Nonwhite n = 66 / 14% of voters

54%

37%

9%

White, college grad 204 / 37%

48%

46%

7%

White, not college grad 222 / 46%

42%

48%

10%

Education

Dem.

Rep.

Und.

H.S. Grad. or Less n = 65 / 18% of voters

43%

42%

15%

Some College Educ. 191 / 35%

44%

48%

7%

4-year College Grad. 143 / 29%

45%

48%

7%

Post-grad. 100 / 16%

54%

39%

8%

Party

Dem.

Rep.

Und.

Democrat n = 145 / 28% of voters

91%

5%

4%

Republican 138 / 28%

7%

88%

5%

Independent 198 / 40%

44%

42%

14%

Another party 15 / 3%

18%

62%

20%

Primary vote

Dem.

Rep.

Und.

Democratic n = 179 / 31% of voters

83%

10%

6%

Republican 170 / 33%

8%

85%

7%

Other 156 / 35%

48%

39%

13%

Intention of voting

Dem.

Rep.

Und.

Almost certain n = 352 / 72% of voters

46%

45%

9%

Very likely 120 / 24%

45%

48%

7%

Somewhat likely 23 / 4%

54%

35%

11%

Not very likely 2 / 0%

—

—

100%

Not at all likely 8 / 1%

—

17%

83%

Percentages are weighted to resemble likely voters; the number of respondents in each subgroup is unweighted. Undecided voters includes those who refused to answer.

This survey was conducted by The New York Times Upshot and Siena College.

Data collection by Reconnaissance Market Research, M. Davis and Company, the Institute for Policy and Opinion Research at Roanoke College, the Survey Research Center at the University of Waterloo, the University of North Florida and the Siena College Research Institute.